Adenylate kinase (EC 2.7.4.3) (AK) [1] is a small monomeric enzyme that catalyzes the reversible transfer of MgATP to AMP (MgATP + AMP = MgADP + ADP). In mammals there are three different isozymes:

• AK1 (or myokinase), which is cytosolic.
• AK2, which is located in the outer compartment of mitochondria.
• AK3 (or GTP:AMP phosphotransferase), which is located in the mitochondrial matrix and which uses MgGTP instead of MgATP.

The sequence of AK has also been obtained from different bacterial species and from plants and fungi.

Two other enzymes have been found to be evolutionary related to AK. These are:

• Yeast uridylate kinase (EC 2.7.4.-) (UK) (gene URA6) [2] which catalyzes the transfer of a phosphate group from ATP to UMP to form UDP and ADP.
• Slime mold UMP-CMP kinase (EC 2.7.4.14) [3] which catalyzes the transfer of a phosphate group from ATP to either CMP or UMP to form CDP or UDP and ADP.

Several regions of AK family enzymes are well conserved, including the ATP-binding domains. We have selected the most conserved of all regions as a signature for this type of enzyme. This region includes an aspartic acid residue that is part of the catalytic cleft of the enzyme and that is involved in a salt bridge. It also includes an arginine residue whose modification leads to inactivation of the enzyme.